Most grinding operations, such as weld seam grinding and deburring of large castings, require a large workspace and dexterity with many degrees of freedom. Industrial robots appear to be suitable for these tasks, however, present day robots and manipulators in general have several technical problems that have prevented their successful application in the process of grinding and like processes. As a result, success in automating these tasks has been limited and many grinding applications have remained highly labor intensive in industry despite low productivity, high costs and hazardous working environments.
In grinding applications, the robot manipulator is required to locate and hold the grinding tool in the face of large, vibratory forces which are inherent in the grinding process. Exposure to these unpredictable loads generally results in large deflections at the tip of the robot arm. These deflections degrade the process accuracy and the surface finish. In addition, the large vibratory loads may cause damage to the robot's mechanical structure.
In conventional machine tools, large deflections are eliminated by designing for maximum stiffness in the whole structure. Unfortunately, it is not feasible for robot manipulators to have such high stiffness. For many robot applications including surface grinding the demands for wide workspace, dexterity and mobility with many degrees of freedom introduce kinematic constraints which make robots unavoidably poor in structural stiffness compared to conventional machine tools.
The technical literature is replete with proposed solutions:
As an alternative to high stiffness design, active feedback control has been applied to grinding robots for reducing dynamic deflections. One active control idea was proposed in which actuators are commanded to increase torques in the opposite direction to the deflections. This method reduces dynamic deflections in a certain frequency range. Generally, it is difficult for this control method to perform well over a wide frequency band because it must drive the entire, massive robot arm.
Actively controlling wrist joints or local actuators which are located near the tip of the robot arm is easier and more effective than moving the whole arm, because the inertial forces are smaller. An active isolator has been applied to a chipping robot, where the isolator attached to the arm tip reduced the vibrations seen by the robot. A multi-axis local actuator was developed which compensates for positioning errors at the end point, in a limited range.
For certain applications the stiffness of the robot can be significantly increased by directly contacting the workpiece. Tool support mechanisms have been developed which couple the arm tip to the workpiece surface and bear large vibratory loads. These mechanisms allow the robot to compensate for the tolerancing errors of the workpiece, as well as to increase the stiffness with which the tool is held. A local support mechanism has been applied to a drilling robot for part referenced positioning.
Thus, a number of methods for improving performance and positioning accuracy have been developed, which can be used for a variety of machining applications. A key to successful application, however, is a sound understanding of the machining process, specifically the dynamic interactions between the tool and the robot manipulator. The grinding process, in particular, is a complicated dynamic process in which nonlinear and coupled dynamic behavior has a direct effect on the surface finish and accuracy.